Delete selected item (which may be null) in external item picker

Question

I have an external list with an edit form that uses an external item picker for foreign key values. The value may be null in the data source, which is also represented in the list. But if I select an item and try to remove it (set it to null again), this does not seem to work. The data source is not changed, the values remains the same as before. Do I overlook something?